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NETRA: Frequently Asked Questions


Vitor F. Pamplona, Ankit Mohan, Manuel M. Oliveira, Ramesh Raskar

Camera Culture Group, MIT Media Lab

Contents

What does NETRA mean?

NETRA means "eye" in Sanskrit, and stands for Near Eye Tool for Refractive Assessment.

What is required?

NETRA is very portable. It needs only two items. (i) A cellphone with eye-test software loaded onto the phone (or downloaded from an app store) (ii) A plastic eyepiece.

What are the steps in the eye-test with NETRA?

The NETRA tool is a small, plastic eyepiece clipped to the front of a cellphone's screen. The patient looks at the LCD screen through this eyepiece and sees a few parallel lines. The patient presses the phone's arrow keys until the parallel lines just overlap. This is repeated eight times, with the parallel lines at different angles. The whole process takes less than two minutes, at which point software loaded onto the phone provides the prescription data.

What NETRA is not?

  • NETRA is not a replacement for your optometrist. It is a measurement device and does not issue prescriptions.
  • NETRA is NOT based on the cell phone camera. It does not use any camera. You probably have seen dozens of beautiful recent inventions where the cell phone camera is used as a replacement for an image sensor in a high end instrument. NETRA does not fall into this category and is a completely new procedure.
  • NETRA is NOT based on reading charts. You probably have seen dozens of beautiful recent inventions where the 2D cell phone display is used as a replacement for passive screens or instructions. NETRA does not fall into this category and instead exploits the high resolution LCDs to create a higher dimensional display and to compensate for human eye aberration.

What is the population statistics for uncorrected refractive error (URE)?

More than two billion people worldwide have refractive error. Very few have access to quality eye-care because existing solutions require a trained optometrist or expensive equipment [VISION2020 Report, Holden2007]. This impacts the developing world in a significant way:

  • 517 million have uncorrected near-vision impairment affecting daily livelihood.
  • 153 million have uncorrected far-vision impairment (affecting 2% of the world population).
  • Uncorrected Refractive Errors are the 2nd leading cause of blindness* globally. 87% of those affected live in the developing world.
  • For many children, hyperopia may remain undiagnosed, leading to undue stress and headaches.

(WHO definition for blindness: vision worse that 3/60 in the better eye.)

Even in US the numbers are very large.

  • Undetected vision problems are the number one childhood handicap. (American Optometric Association)
  • An estimated 10 million children suffer from undetected vision problems. (National Parent Teacher Association)
  • 60% of students identified as "problem learners" have undetected vision problems. (American Optometric Association)

What is the SIGGRAPH 2010 paper about?

The paper describes the NETRA technique, the optical setup, analysis of various display patterns, proof of concept using several prototypes, and appropriate validation studies and user tests. Paper authors are: 

  • Vitor F. Pamplona, Visiting Ph.D. student at MIT Media Lab, Camera Culture group.
  • Ankit Mohan, Postdoctoral Associate at MIT Media Lab, Camera Culture Group.
  • Manuel M. Oliveira, Visiting Associate Professor at MIT Media Lab, Camera Culture Group.
  • Ramesh Raskar, Associate Professor at MIT Media Lab, Camera Culture Group.

What do you recommend as background reading?

How is the refractive error measured? What is a diopter?

Refractive error is measured in diopters, which represents the refractive power a lens required to correct the aberrations of the eye. Doctors typically prescribe glasses in steps of 0.25 diopters. Myopia (blurry far sight) required corrective lenses with negative diopters, and hyperopia (blurry near sight) requires lenses with positive diopters.

What is astigmatism and how is it measured?

Astigmatism refers to optical aberrations of the eye that are not symmetric about the optical axis. Astigmatism in eyes is approximated by a spherical power, and an additional cylindrical power. Spherical and cylindrical powers are measuremed in meridians oriented 90 degrees apart. The axis of astigmatism is the axis of the spherical power.

How it Works

How does NETRA compare to a Shack-Hartmann Wavefront Aberrometer?

Devices using the Shack-Hartmann sensor shine a laser into the eye of the patient, are quite expensive, and require a trained professional operator. The NETRA system uses the dual of a Shack-Hartman sensor, and replaces the laser with simple user interaction.

Shack-Hartman sensor measures the displacement of the spot to estimate local slope of the wavefront (while using adaptive optics to maintain the sharpness of the incident laser beam). NETRA gets around the need for coherent light source and focussed beams. In simple words, our method asks the user to interactively move each spot till all spots are aligned on the retina. The spot movements compensate for the wavefront aberrations present in the human eye. (However, this analogy does not extend to astigmatism and other higher order effects. The search space explodes even for a minor astigmatism. In addition, aligning a dozen spots sequentially will be cumbersome for the user.) We exploit the fact that aberrations can be expressed using only a few parameters. So we guide the user to navigate in this low dimensional space.

The human interaction in a low dimensional parameter space is the key to NETRA. This significantly reduces the cost of the device and makes it appropriate for self-evaluation, while still providing comparable data.


How can an LCD display perform wavefront compensation?

Coherent light sources, like lasers, are ideal for wavefront analysis, sensing and compensation. LCD displays do not produce images with coherent light. So LCDs may appear to be limited in their ability to compensate for human eye aberrations. Nevertheless, by introducing phase–sensitive optical elements, we can exploit the phase information relevant to our problem here. See our recent work in augmenting ray representation via Augmented Light Fields to analyze wave phenomena.

What does the micro-lens array in front of the LCD actually do?

The micro-lens array in front of the LCD essentially creates a 4D display. A traditional 3D display presents a slightly-different view to a person’s left eye and their right eye. With NETRA, we present different views through different parts of the same eye.  If the eye is free from aberrations, it focuses clearly and these disparate views overlap perfectly. But far and near-sighted users have to press a few buttons to make that happen. The number of tweaks it takes for them to hit that ideal alignment reveals the correction they need to focus clearly.

Why do you use an alignment task to estimate refractive error?

Alignment is an easier task than discerning blur for humans. Humans can perform the task of aligning lines with remarkable accuracy. Using an alignment task rather than blur estimation improves the resolutoin of our refractive error estimates.


Why hasn't this idea been explored before?

NETRA relies on a programmable high resolution display to generate patterns at arbitrary depths. The resolution of cheap LCDs found on popular phones recently took a giant leap. For instance, while current LCD monitors support about 90 Dots-Per-Inch (DPI), the Nexus One and the new iPhone 4G supports 250 and 326 DPI respectively [1,2]. The display resolution defines the resolution of the NETRA device in diopters, and for it to be usable as an optometry solution, reasonably high resolution displays are required.

How did you come up with this idea?

The NETRA optics is essentially an array of Bokodes, a previous project at the MIT Media Lab. The NETRA project came about by bringing together the experiences in novel optics and cameras from the MIT Media Lab (Ankit, Ramesh), and the expertise and interests in the human eye from UFRGS Instituto de Informatica (Vitor, Manuel).

How much does the device cost?

Our current prototype cost less than USD 2.00 using laser-cut, hand-assembled parts. Since the device essentially made of plastic, this cost will rapidly reduce to just a few cents if mass produced.

What is the resolution in diopters of the current prototype?

The resolution is 0.4 diopters using the Nexus One device (focal length 30mm). The Apple iPhone 4G, with the new Retina Display should achieve a resolution of approximately 0.28 diopters (focal length 30mm). For measuring eye correction, the average absolute errors from the known prescriptions were under 0.5 diopter (σ = 0.2) for both cylindrical and spherical powers. The average absolute error of our estimates of the cylindrical axis was under 6 degrees. Optometrists typically prescribe in multiples of 0.25 diopter, and 10 degrees axis.

More info about high resolution displays: [1,2]

Miscellaneous

Does NETRA replace optometrists?

No. Our device can be thought of as a thermometer for visual performance. Just as a thermometer measures corporal temperature and does not prescribe medicine, NETRA measures the refractive error and does not necessarily prescribe glasses. NETRA allows a user to self-assess the performance of her eye over time. The goal of NETRA is to empower people, not replace optometrists. We expect the self-awareness will encourage more people to visit an optometrist where available.

Does NETRA use the cell phone camera?

No. NETRA does not use any electronic sensor. We use only the high resolution display. And the natural high resolution of the human retina as a sensor.

Why is high resolution display so important? What is the relation pixel size vs dioptres?

The resolution of the display screen contributes to the accuracy and resolution of the measurements in dioptres provided by the device. There is a linear relationship between pixel size and the measurement resolution in diopters.

Does NETRA work with low end phones?

We are currently working on expanding the range of phones that NETRA works on. We are also investigating novel pattern designs which may provide adequate resolution with low resolution displays.

What are other efforts in eye-care in developing counties?

There are basically 3 components to refractive eye-care: (#1) measurement, (#2) making eye-glasses and (#3) deployment. Although there are many efforts, there is surprisingly little work in measurement using a portable, low-cost device.

Here are others solving problem #2 and #3. We are delighted to have ongoing discussions with many visionaries and organizations in the field


However, we are solving the first piece of the puzzle.

Q: Where can I get NETRA?

You can download the software apps for your mobile phone here. Android: Android Market (Just the tutorial right now) iPhone: (coming soon) Pl note these applications are minimal because they require the clip-on eyepiece to be useful.

Q: Where can I get the clip-on eyepiece?

We are not ready to market the eyepieces yet. Join our mailing list at http://groups.google.com/group/mitnetra/ anonymously to stay tuned.

Q: I am affiliated with a non-profit center for eye-care. How can I participate?

We are not ready to send the eye-pieces yet. But please send us an email describing your scenario. Also join our mailing list at http://groups.google.com/group/mitnetra/ to participate in the conversation.

Media Coverage

Coverage in Video

Coverage in Audio

English

Coverage in India

Coverage in Brazil

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